American democracy’s comic opera frequently features collaborations of “bootleggers and Baptists.” These entertainments are so named because during Prohibition, Baptists thought banning Demon Rum would improve public morals (oh, well) and bootleggers favored the ban because it made scarce a commodity for which there was a demand that they could profitably supply. On Monday, the Supreme Court will listen — with, one hopes, a mixture of bemusement and amusement — to arguments concerning another prohibition.

This one concerns a law banning what many millions of Americans do anyway — illegally betting between $150 billion and $400 billion annually on sports events. Illegality prevents precise knowledge, but if the sum is just $150 billion, that sum exceeds the combined revenue of Microsoft, Goldman Sachs and McDonald’s. The Baptists in this case are those who consider gambling a vice that state governments should discourage. The bootleggers are those who supply illegal gambling services on the Internet and elsewhere.

The court’s nine fine minds need not, and should not, trouble themselves with the question of whether this particular prohibition is sensible. They should, however, defend federalism by telling the national government to stop telling state governments what laws they cannot change.